A typical electromagnetic solenoid includes a wire coil wound round a stationary electromagnetic pole, and a movable ferromagnetic core (armature) which is separated by an air gap from the stationary electromagnetic pole. A surrounding soft iron frame forms, along with the stationary pole and the movable core, a magnetic circuit which is locally interrupted by the air gap. Upon excitation of the coil by an electric current, a magnetic flux is generated in the magnetic circuit. The magnetic flux results in the movable core being attracted to the stationary pole. The movable core, usually in the form of a plunger, is held in a retracted state until the coil is de-energized, whereupon it is then separated from the stationary pole by gravity if applicable, or by a biasing spring.
When the energized state must be maintained for an extended period of time and the energy source is limited, a magnetic latching solenoid is preferred. A magnetic latching solenoid uses residual magnetism or further includes a permanent magnet as part of the magnetic circuit and has extended and retracted natural states. In the extended state, the air gap is maximal and the magnetic flux generated by the permanent magnet or residual magnetism is too weak to pull the plunger against the biasing spring.
In order to throw to the retracted state (latch), a pulse of electric current is applied to the coil, generating an additional magnetic flux sufficient to stress the biasing spring and pull the plunger to the retracted state wherein the air gap is zero. In the retracted state, the flux density generated by the permanent magnet is sufficiently concentrated to latch the plunger against the stationary pole whilst stressing the biasing spring. The usable portion of the force applied by the permanent magnet in the retracted state is often called the holding force of the latching solenoid, while the pulling force generated by the electric pulse at the start of the stroke is named the attracting force.
In order to unlatch the solenoid, a reverse polarity pulse of electric current is to applied to the coil, sufficiently reducing (canceling) the holding force of the permanent magnet such that the biasing spring can force the plunger away from the stationary pole.
In applications where space is limited, it may be desirable that the solenoid be exactly matched to its duty in terms of force and electric power. In such cases it is important to find the minimal size of permanent magnet and coil that will provide the required holding and attracting forces respectively.
The attracting and holding force of a solenoid in simple terms is approximately defined by the equation:F=AB2/2μ0 where: F is the force obtained,
A is the cross sectional area of the plunger,
B is the flux density generated at the plunger face, and
μ0 is the permeability of free space.
However, the flux density B, cannot be increased indefinitely as the core material enters into saturation at a certain flux density level related to the core material. The flux density B depends also on the air gap or stroke of the solenoid. As a general rule, the attraction force of a magnet is inverse to the square of the gap.
Accordingly in the search for maximal attraction and holding forces, the flux density B must be close to saturation and the cross-sectional area A, reduced to the point where that desired flux density is obtained. Reduction of the cross-sectional area at the entire length of the plunger and stationary pole is not recommended since the reluctance of the magnetic circuit increases. Thus it is known in the art to provide a conical or stepped-conical saturation tip to the plunger in order to maximize the force of attraction of long stroke solenoids. Short stroke solenoids, however, are typically known to act better with a flat plunger face.
References to such prior art can be found in U.S. Pat. Nos. 6,698,713; 7,280,021; 6,392,515; 5,915,665; 3,805,204 and U.S. Application 2009/0072636.
Although latching solenoids are operated by a DC pulse, the short duration of the excitation pulse is equivalent in behavior to an AC current. Accordingly, an additional consideration factor is involved, similar to the known characteristics of a ferromagnetic core which is energized by high frequency AC current. It has been found that in such applications, the magnetic field produced by the eddy current and displacement current due to the electrical field will shield the magnetic flux from the inner portion of the core cross-section. This results in a flux skin effect analogous to the skin effect in the conductors of a wound coil. Simply stated, the flux density during excitation of the coil is higher at the circumference of the plunger face than at the inner surface.
A disadvantage of the suggested conical saturation tip for long stroke solenoids or the flat face for short stroke magnetic latching solenoids is related to the skin effect that directs the majority of the flux lines to the exterior of the plunger volume, thus resulting in early saturation of the effective surface area and loss of mechanical power.
Additionally in short stroke magnetic latching solenoids with a flat plunger face the desired stroke may not be sufficiently large and the permanent magnet may retract the plunger back after the reverse polarity pulse of electric current is applied to the coil.
Consequently a new approach is required to further improve the efficiency of magnetic latching solenoids, particularly of short stroke magnetic latching solenoids.